This invention relates to a process for treating spent hydrochloric acid from a galvanizing acid or pickling bath, in which chemical changes occur causing removal or certain undesirable contaminants as well as conversion of the acid into a material usable in the preflux section of the galvanizing system. In one specific aspect, it relates to a process for treating spent hydrochloric acid and forming therefrom two useful products, one being ammonium chloride which may be recycled to the galvanizing preflux section, and the other being magnetite which is useful as a paint pigment, or as an addition in cattle feed.
In conventional galvanizing systems, the metal to be coated undergoes five distinct treatments. Initially, the metal is passed through a caustic bath where oils and greases are eliminated and the metal is thoroughly washed clean. A typical caustic used in this first stage bath is sodium hydroxide. From the first stage caustic bath the cleaned metal is passed to a second stage water rinse bath where caustic from the first bath is removed. The third stage bath in the process commonly contains an acid such as hydrochloric acid. This third stage bath is termed the pickling bath. It is in this third stage bath that the surface preparation of the metal to be galvanized occurs. Thus the pickling bath removes iron oxide from the metal object to be galvanized. From the pickling bath, the metal object passes to a fourth stage bath termed the preflux bath. This bath conventionally contains a solution of ammonium chloride and zinc chloride, and in this preflux bath the metal object undergoes secondary surface preparation where ammonium chloride and zinc ammonium chloride acting as a wetting agent are added to the surface of the metal object. The fifth and final conventional treatment bath in the galvanizing process is the galvanizing tank commonly containing molten zinc for application as a coating on the metal object having the surface thereof prepared as previously described in the initial four treatment stages.
Heretofore, it has been common in conventional galvanizing systems for the hydrochloric acid in the pickling bath to become spent or unusable. Thus, after many hours of operation, the pickling acid increases in iron concentration and increases in water concentration. When the iron concentration for example reaches approximately 9% by weight, and when the increase in water concentration drops the Baume of the pickling acid to 10.degree.-12.degree., it has been necessary to drain and dispose of the spent acid in the pickling tank, and to recharge the pickling tank with fresh acid. This has resulted in increased expenditures for draining and recharging and has presented spent acid disposal problems. Thus, in conventional galvanizing systems, the spent acid has been drained from the pickling bath and pumped to a tank truck for transportation and ultimate disposal by means which, according to present day environmental regulations, are prohibited. Hence, previous attempts to deal with the problem of spent pickling acid have not only proved to be unsuccessful but also laborious and time consuming.